This invention relates to a method of degassing an aqueous solution for the prevention of coating troubles in a process of delivering the aqueous solution and applying it to a web.
In general, when a coating solution containing dissolved air or bubbles is applied to a web, it is difficult to form a uniform coating membrane because of the occurrence of longitudinal streaks, pinholes and the like. Thereupon, in the case of precise coating membranes, the coating solutions therefor need to remove dissolved air and bubbles from them prior to coating.
Heretofore, various degassing methods have been proposed. For example, the degassing method disclosed in Japanese Patent KOKAI 6-335623 comprises using a composite membrane which is a hollow fiber composed of a porous membrane (4-methylpentene-based polymer) and a nonporous membrane. Japanese Patent KOKOKU 51-35259 discloses a continuous degassing apparatus comprising introducing a high viscosity liquid into a reduced pressure chamber, reducing the viscosity temporarily by heating, streaming it on a wetting wall having a surface area gradually increasing from the top to the underside in a tower to decrease membrane pressure, and degassing by sucking the reduced pressure chamber by a vacuum pump. Japanese Patent KOKAI 64-38105 discloses a treating of silver halide emulsion for photograph which comprises passing the emulsion containing dissolved air and microbubbles on the outside of a porous polymer membrane tube to be stored in a jacket, and removing the dissolved air and microbubbles while sucking the inside of the tube and pressuring the emulsion. Japanese Patent KOKAI 9-225206 discloses a degassing apparatus using a membrane of tetrafluoroethylene-hexafluoropropylene copolymer. Thus, degassing apparatus was developed in order to solve a problem of a conventional degassing apparatus using a porous polymer membrane in degassing solvent solutions, oils, fats, and solutions containing surfactant which tends to permeate liquid to wet, and thereby, loses degassing ability.
However, when a high viscosity aqueous coating solution is degassed by the degassing under reduced pressure, microbubbles are generated in the solution. Since the coating solution has a high viscosity, the generated microbubbles cannot be separated by flotation, and are delivered to a coater to cause coating troubles. The degassing apparatus using a membrane of tetrafluoroethylene-hexafluoroethylene copolymer were developed for degassing solutions having affinity to porous polymer membranes such as organic solvent solutions, oils, fats, surfactant solutions, and the patent application is quite silent about high viscosity solutions and solutions containing a water-soluble polymer.